Additional relevant info:
A similar issue was also discussed three years ago on Sun Jun 17:15:54 CEST 2015 and was about his surround sound setup, but did not touch on the external microphone problem: http://mailman.alsa-project.org/pipermail/alsa-devel/2015-June/093317.html
alsa-info.sh: http://www.alsa-project.org/db/?f=1d8616ba5977308e03db6c3a86e36e9e9b38d6f0
Graph of ALC668 chipset from hdaanalyzer: http://i.imgur.com/c08DNJW.png
setting alsa-mode[1-8] does nothing to help the issue.
There are several of similar bug reports around the web experiencing similar issues, and on different distros.
Microphone works perfectly in windows
I have a ASUS ROG G751JT, but this problem seems to happen with all laptops under the G751 name.
These are my kernel configs: https://paste.pound-python.org/show/GRk4T6xNvmdp8W7K9slJ/
Thank you! -Håvard